Coal fired power plants face increasingly stringent pollution control standards for acid gases and trace air toxics, including sulfur dioxide (SO2), sulfur trioxide (SO3), and mercury (Hg). This requires current best control practices for sorbent pollution control processes to be improved. Commonly, plants utilize control practices such as activated carbon injection (ACI) and dry sorbent injection (DSI). In many cases, however, further increase in sorbent injection rate is uneconomical, ineffective, or adversely impacts the pollution control process. Additionally, use of DSI for acid gas control can cause severe performance degradation in electrostatic precipitators, leading to challenges in particulate emission control.
A common approach to mercury control involves injecting a sorbent in the form of a carbonaceous material, such as powdered activated carbon, for mercury sorption. Acid gases (including SO2) are controlled through DSI, with common sorbents including trona, hydrated lime, and sodium bicarbonate. Finally, SO3 is commonly used as an ESP ash conditioning agent—for maintaining electrical and opacity performance—to maintain permitted particulate emissions levels.
Used in conjunction, however, the various sorbents can interfere with the performance of other sorbent(s), thereby reducing sorbent performance. By way of example, sulfur trioxide (SO3), while necessary for particulate emission control, is detrimental to mercury capture with most carbon sorbents, including powder activated carbon (PAC). This effect is well documented in industry testing. SO3 is captured with DSI, which is intended to target SO2 in the flue gas. While this allows carbon sorbents to perform better at mercury control, it also removes beneficial SO3 that otherwise would be used for ESP conditioning. This results in degraded ESP performance and particulate emission control.
Accordingly, a solution is required that will allow for a combination of standard emission control technologies while maintaining multiple contaminant emission targets. The solution must maintain particulate collection and acid gas control targets while satisfying mercury removal targets (MATS compliance), especially in units utilizing carbon injection for mercury control.